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Bipolar planetary nebulae (BPNe) possess a broad range of shapes, ranging from narrow-waisted butterfly-like structures, through to those which are more nearly cylindrical. We point out that these morphologies appear to be correlated with radio surface brightness T b , in the sense that higher values of T b are associated with envelopes having narrower waists. If one interprets the variation in T b as arising from shell evolution, as is usually assumed for other planetaries, this would thendoi:10.1051/0004-6361:20020031 fatcat:tczn7ccymfakdex3annvvy4ao4